This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A central question in the study of cell proliferation is, what controls cell- cycle transitions. Although the accumulation of mitotic cyclins drives the transition from the G2 phase to the M phase in embryonic cells, the trigger for mitotic entry in somatic cells remains unknown. We report that the synergistic action of Bora and the kinase Aurora A (Aur-A) controls the G2-M transition. Bora accumulates in the G2 phase and promotes Aur-A-mediated activation of the Polo-like kinase 1 (Plk1), leading to the activation of the cyclin-dependent kinase 1 (Cdk1) and mitotic entry. Mechanistically, Bora interacts with Plk1 and controls the accessibility of its activation loop for phosphorylation and activation by Aur-A. Thus, Bora and Aur-A control mitotic entry, which provides a mechanism for one of the most important, and yet ill-defined events in the cell cycle.